Magnesium as a component of a range of enzymatic systems is a very important intracellular cation in the organism. Its monitoring is limited in many observations only to determination of its concentration in blood serum. We have done an analysis of serum and erythrocyte magnesium concentrations in 23 healthy women, 70 healthy men and in 184 patients with ulcerative disease of gastroduodenum. It was proved in all the monitored groups that erythrocyte concentrations of magnesium were lower compared to values which were estimated from serum concentrations in 9 from 23 healthy women, in 3 from 60 healthy men, in 10 from 101 ill men with ulcerative gastroduodenal disease, and in 17 from 83 women with ulcerative gastroduodenal disease. The highest rate of low concentrations of erythrocyte magnesium in healthy women with physiologic concentrations of magnesium in serum was also confirmed by currently created subgroup of healthy women (n = 11) who undergone together with other analysis peroral Mg2+ load test. This test confirmed magnesium deficiency in 10 from 11 women. The results showed there are more frequent deficiencies of magnesium in organisms then it is generally assumed. They also proved the importance of nutrition and regular food in population of healthy, young women.